1988 Oldsmobile Delta 88 High Idle

The Delta 88 is quite the transmission replacement project. Before the breakdown, she began sputtering as if out of gas, but only around 50 mph. After about 200 more miles she would not go into gear 80% of the time. She has always had a high idling problem. I'm wondering if changing the transmission as well as some cleaning may fix the idle problem. If not what can I do?